1. Field of the Invention
The present invention relates to a coplanar line filter and duplexer, more particularly to a coplanar line filter and duplexer for use in a microwave band communications device and the like.
2. Description of the Related Art
In recent years, a bandpass filter using a coplanar resonator has been proposed as a filter in a microwave band communications device. For instance, FIG. 10 shows a bandpass filter 81 comprising .lambda./4 coplanar resonators Q11.about.Q13 are connected in series. The .lambda./4 coplanar resonators Q11.about.Q13 are connected between input and output terminals 87 and 88 via capacitors C11.about.C14, comprising lumped constant elements. The .lambda./4 coplanar resonator Q11 comprises a center conductor 82a and a ground conductor 83, provided while ensuring a gap from the center conductor 82a. One end of the center conductor 82a is electrically connected to the ground conductor 83, forming a .lambda./4 coplanar resonator Q11 with one connected end. Similarly, the .lambda./4 coplanar resonators Q12 and Q13 comprise center conductors 82b and 82c, having electrical length corresponding to a quarter wavelength, and the ground conductor 83, provided while ensuring a gap from these center conductors 82b and 82c.
Furthermore, the bandpass filter 91 shown in FIG. 11 comprises .lambda./2 coplanar resonators Q14.about.Q16 connected in series. The .lambda./4 coplanar resonator Q14 comprises a center conductor 92a, having electrical length corresponding to a half wavelength, and ground conductors 93, provided on either side of the center conductor 92a while ensuring a gap between the center conductor 92a and the ground conductors 93. Similarly, the .lambda./2 coplanar resonators Q15 and Q16 each comprise center-conductors 92b and 92c, having electrical lengths corresponding to a half wavelength, and the ground conductors 93, on either side of the center conductors 92b and 92c while ensuring a gap between these and the ground conductors 93. The .lambda./2 coplanar resonators Q14.about.Q16 are connected in series by capacitive couplers C16 and C17, formed at a gap provided between center conductors 92a and 92b and a gap provided between center conductors 92b and 92c, and are connected between input/output terminals 97 and 98 by capacitive couplers C15 and C18, formed at a gap provided between the center conductor of the input/output terminal 97 and the center conductor 92a of the resonator Q14, and a gap provided between the center conductor of the input/output terminal 98 and the center conductor of the resonator Q16.
However, in the bandpass filter 81 shown in FIG. 10, since the center conductors 82a.about.82c of the .lambda./4 coplanar resonators Q11.about.Q13 are mutually separated by the ground conductor 83, it is difficult to connect the .lambda./4 coplanar resonators Q11.about.Q13 with a distribution-constant device, and design was complex. On the other hand, since the bandpass filter 91 shown in FIG. 11, uses center conductors 92a.about.92c having electrical lengths corresponding to a half wavelength, it is large-scale by comparison with a bandpass filter which used .lambda./4 coplanar resonators.